Wind power fed network

ABSTRACT

An electric network for generation and transmission of electric power, including a power generating part, a point of common connection for the power generating part, a transmission link, a load network, and a reactive power compensator. The transmission link is coupled between the point of common connection and a grid connection point at the load network. The reactive power compensator is coupled to transmission link. The power generating part includes at least one wind turbine with an electric generator of induction type, coupled to the point of common connection. The reactive power compensator includes a capacitor bank and in parallel coupling to the capacitor bank a controllable inductor having a magnetic core, a main winding for alternating current, and a DC-control winding for direct current. The DC-control winding for control of the magnetic flux is set up by the main winding via orthogonal magnetization of the core.

TECHNICAL FIELD

The present invention relates to the use of a reactive powercompensating means for compensation of reactive power in an electricnetwork having a power generating part, a point of common connection forthe power generating part, a transmission link, a load network, thetransmission link coupled between the point of common connection and agrid connection point at the load network, wherein the reactive powercompensating means is coupled to the transmission link.

It also relates to an electric network for generation and transmissionof electric power, having a power generating part, a point of commonconnection for the power generating part, a transmission link, a loadnetwork, and a reactive power compensating means, the transmission linkcoupled between the point of common connection and a grid connectionpoint at the load network, and the reactive power compensating meanscoupled to the transmission link.

BACKGROUND ART

A general description of wind power generation is to be found forexample in the article Blaabjerg and Ned Mohan: Wind power, WileyEncyclopedia of Electrical and Electronics Engineering, John Wiley &Sons, 1999, volume 23, pages 613-618, which article is herebyincorporated by reference.

A wind power plant usually comprises a plurality of windmills, eachcomprising a wind turbine mechanically coupled to an electric generatorfor conversion of the wind power to electric power. The wind turbinesare, in dependence on the local wind conditions, distributed over agiven area, typically in a number of parallel strings perpendicular tothe prevailing wind direction, or where no such wind direction is to befound, in a grid layout.

A power collection system within the wind power plant is formed bycoupling the generators along a string to a radial cable running alongthe string and connecting the radial cables to each other at a so calledpoint of common connection (PCC).

The power generated by the wind power plant is supplied to a loadnetwork in the form of an electric power grid, for example a utilitygrid, having a rated frequency (usually 50 or 60 Hz) and a rated voltagethat may typically be at the 132 kV level. Typically, the rated voltageat common connection is 22 kV and the point of common connection is thencoupled to the power grid via a high voltage step-up power transformer.

Windmills may be divided into two categorises, i.e. fixed-speed andvariable-speed mills, referring to whether the turbine and the rotor ofthe electric generator will operate at an at least substantially fixedrotational speed, determined by the frequency of the power grid, oroperate with a variable rotational speed adapted to the actual windconditions and the characteristics of the wind turbine.

Fixed-speed windmills may be equipped with some kind of synchronousgenerators, such as reluctance machines or conventional synchronousmachines, but are, due to mechanical design considerations, more oftenequipped with induction generators.

Induction generators are of an uncomplicated design requiring only aminimum of control equipment, which also makes them attractive from aneconomical point of view. As they are usually designed with a low numberof poles, typically 4 or 6, a mechanical gearbox is required to adaptthe low rotational speed of the wind turbine to the speed of thegenerator.

The control equipment usually comprises only some starting equipment tolimit the inrush current when the generator is connected to the powercollection system.

However, induction generators cannot inherently generate reactive power,and the reactive power needed for their operation is thus provided byphase capacitors coupled to the stator windings of the generator.

The reactive power consumption of this type of generators is notcontrollable but dependent on the active power and on the voltage of thegenerator. Consequently, the exchange of reactive power with the grid towhich the generator is coupled will vary substantially in dependence onthe load of the generator, and the voltage of the network will exhibitcorresponding voltage variations. These voltage variations areparticularly considerable when the network is weak, i.e. has a low shortcircuit capacity.

The operator of the electric power grid usually has a requirement on themaximum level of the voltage supplied from the wind power plant. Usuallyall the generated electric power is supplied to the grid. In particularwith increasing unit sizes of the windmills and with an increasingdistance between the wind power plant and the grid, the voltage controlat the point of common connection has been identified as a problem. Thevoltage rise, typically occurring at times of low grid load and highoutput power from the windmills, is dependent on the short circuit powerat the point of common connection and in particular where the wind powerplant is equipped with fixed-speed windmills, a situation may arisewhere it will be necessary to switch off a windmill in order to keep thevoltage level within prescribed limits. This of course means anundesirable loss of energy.

Thus, in order to obtain an acceptable voltage control in the network,in particular when the network is weak, a controllable reactive powercompensation means is required.

The mechanical torque of the wind turbine is subject to fluctuations, inparticular to periodic fluctuations due to the design of the windturbine, typically at a frequency in the order of 1-2 Hz, occasionallyeven below 1 Hz. A predominant source of such fluctuations is theso-called vortex interaction. However, for example imperfections in thegearbox may be the cause of fluctuations even in higher frequencyranges, typically in the order of up to 8 Hz.

Although the induction generators of a fixed-speed windmill have someinherent damping, such torque fluctuations will, due the consequentialfluctuations in the rotational speed of the induction generator, causefluctuations in the outputted active power of the generator, and, due tothe inherent characteristic of such a generator, also in the reactivepower exchange with the power collection system, thereby affecting thevoltage quality of the electric power grid.

To obtain a control of reactive power flow that is fast enough to reducevoltage variations in the above mentioned frequency ranges, the reactivepower compensation means preferably shall to be of the kind comprisingone or more capacitor banks and a controllable inductor coupled inparallel with the capacitor banks.

In a known type of such compensator means, the inductor is connected inseries with gate-controlled thyristors coupled in anti-parallel, wherebythe susceptance of the inductor is controlled by the firing angle of thethyristors, so-called Thyristor Controlled Reactors (TCR).

However, such compensators generate, due to their operational principle,harmonics, which inter alia requires some kind of harmonic filtering toavoid that the harmonic currents are injected into the connected grid.

As mentioned above, in a wind park, a plurality of windmills are coupledto a so-called point of common connection. The voltage at this point ofcommon connection is usually in the range of 10-30 kV. In cases wherethe power has to be transmitted over longer distances along atransmission link, the transmission link is preferably arranged tocomprise a high voltage step-up transformer for increase of thetransmission voltage to the range of 100-500 kV.

This limits the options for connection of a TCR, which, because of thethyristors comprised therein, is usually not connected directly, i.e.without a coupling transformer, to voltages higher than 36 kV.

SUMMARY OF THE INVENTION

It is an object of the invention to provide in an electric networkhaving a power generating part that comprises at least one wind turbinewith an electric generator of induction type, the use of a reactivepower compensating means that can be directly connected even to voltagestypical for transmission links, and the susceptance of which can becontrolled without generation of harmonics.

It is another object of the invention to provide such use in an electricnetwork having a load network which has a short circuit capacity that islower than 10 times the rated power of the power generating part.

It is another object of the invention to provide an electric networkhaving a power generating part comprising at least one wind turbine withan electric generator of induction type, a transmission link, and a loadnetwork, wherein a reactive power compensating means can be directlyconnected even to voltages typical for transmission links, and thesusceptance of which can be controlled without generation of harmonics,

According to the invention these objects are accomplished by providing areactive power compensation means having at least one capacitor bank andin parallel coupling thereto a controllable inductor with a magneticcore, and wherein the susceptance of the inductor is controllable viaorthogonal magnetization of the core.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained in greater detail by description ofembodiments with reference to the accompanying drawing, which isschematic and drawn as a combined block- and single line diagram, onlyshowing main components which are of relevance for the understanding ofthe invention, and wherein

FIG. 1 shows a an embodiment of an electric network according to theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following description relates both to electric network and to theuse of a reactive power compensating means.

FIG. 1 shows a three-phase electric network for generation andtransmission of electric power, having a power generating part 1, apoint of common connection PCC for the power generating part, atransmission link, a load network LN, and a reactive power compensatingmeans 2.

The active power generated by the power generating part is supplied tothe load network via the transmission link, that is coupled between thepoint of common connection and a grid connection point PGC at the loadnetwork.

The transmission link comprises a high voltage step-up transformer Twith its low voltage side coupled to the point of common connection, andits high voltage side coupled to a transmission line W. The transmissionline is at least to a part embodied as a cable CAB.

The power generating part comprises one or more, typically a plurality,of windmills, of which three are illustrated in the FIGURE with thedesignation numbers 11, 12, and 13.

The windmill 11 has a wind turbine 111, and a three-phase squirrel-cageinduction generator 113. The generator is coupled to the wind turbinevia a gear box 112. Electrically, the generator is coupled to the pointof common connection via a step-up transformer 114. Although not shownin the FIGURE, the generator may be equipped with starting equipment aswell with phase capacitors coupled to its stator windings for generationof reactive power during operation

The windmills 12 and 13 are of similar kind as the windmill 11.

The active power output of the windmills may be controlled via a per seknown pitch control system.

Typically, the output voltage of the generator is 690 V, and the voltageat the point of common connection 22 kV. The rated frequency of thenetwork is usually 50 or 60 Hz.

The high voltage step-up transformer T typically has a ratio increasingthe voltage at the transmission line W to 132 kV.

The reactive power compensating means is coupled to the transmissionlink at the point of common connection, and comprises a capacitor bank21 and in parallel thereto an inductor 22 with controllable susceptance.

The over-all generation of reactive power of the reactive powercompensating means is thus controllable to a desired level by reducingthe reactive power generated by the capacitor bank with the controllablereactive power consumption of the inductor.

The inductor has a magnetic core 221, a main winding 222 for alternatingcurrent, and a DC-control winding 223 for direct current. By variationof the current Id fed into the DC-control winding, the magnetic flux setup by the main winding is influenced via orthogonal magnetization of thecore, so-called cross magnetization.

An inductor of this kind is described for example in U.S. Pat. No.4,393,157, which is hereby incorporated by reference.

The voltage UC at the point of common connection is sensed by a voltagesensing device MD and a value thereof is supplied to a controller 23,which in dependence on the deviation between a reference value for thatvoltage and its actual value outputs the current Id in such a way thatthe deviation goes towards zero.

When the geographical distance between the power generating part and theload network is great, and in particular in cases where the powergenerating part is located off-shore, and the transmission line thuswill comprise a sub-marine cable CAB, it might be advantageous to locateat and couple the reactive power compensating means to the gridconnection point.

This alternative localization of the reactive power compensating meansis in the FIGURE indicated with the box labelled 2′.

The reactive power compensating means 2′ is similar to the one describedabove, however, designed for connection to the voltage at the gridconnection point. In this case, a voltage sensing device MD' senses thevoltage at the grid connection point and supplies a value thereof to thecontroller 23.

The following advantages are achieved by the invention.

The harmonics generated by the reactive power compensation means arevery low.

The reactive power compensation means may be connected to voltage levelsthat typically are used in transmission systems

In particular when the load network is weak, which in this context meansthat the load network has a short circuit capacity that is lower than 10times the rated power of the power generating part, a reactivecompensator means of the kind described above will significantly improvethe voltage quality of the electric network.

In situations such as a earth fault somewhere in the network, forexample in the load network, the induction type generators might losetheir magnetization due to the decreased voltage, and as a consequencethereof increase their rotational speed. After clearing of the fault,the voltage will return in the network. In such a situation, reactivepower has to be rapidly supplied to the windmills, which can easily besupplied by a system according to the invention.

It shall be understood that, although not particularly shown in theFIGURE, embodiments of the transmission link without any step-uptransformer, and where it is embodied only as conductors directlyconnecting the point of common connection to the point of gridconnection, are within the scope of the claims.

1. An electric network for generation and transmission of electricpower, comprising a power generating part, a point of common connectionfor the power generating part, a transmission link, a load network, anda reactive power compensating means, the transmission link coupledbetween the point of common connection and a grid connection point atthe load network, and the reactive power compensating means coupled totransmission link, wherein the power generating part comprises at leastone wind turbine with an electric generator of induction type, coupledto the point of common connection, and wherein the reactive powercompensating means comprises a capacitor bank and in parallel couplingto said capacitor bank a controllable inductor having a magnetic core, amain winding for alternating current, and a DC-control winding fordirect current, said DC-control winding for control of the magnetic fluxset up by the main winding via orthogonal magnetization of the core. 2.The electric network according to claim 1, wherein the reactive powercompensating means is coupled to the point of common connection.
 3. Theelectric network according to claim 1, wherein the transmission linkcomprises a high voltage step-up transformer with its low voltage sidecoupled to the point of common connection, wherein the reactive powercompensating means is coupled to said grid connection point at the loadnetwork.
 4. The electric network according to claim 3, wherein the powergenerating part is located off-shore and in that the transmission linkcomprises a sub-marine cable.
 5. The electric network according to claim1, wherein the load network has a short circuit capacity that is lowerthan 10 times the rated power of the power generating part.
 6. Theelectric network according to claim 1, wherein the reactive powercompensating means comprises a controller for generating the directcurrent for said DC-control winding in dependence on a voltage sensed atthe reactive power compensating means.
 7. Use of a reactive powercompensating means for compensation of reactive power in an electricnetwork having a power generating part with at least one wind turbinewith an electric generator of induction type, a point of commonconnection for the power generating part, a transmission link, a loadnetwork, the transmission link coupled between the point of commonconnection and a grid connection point at the load network, and thereactive power compensating means coupled to the transmission link, thereactive power compensating means having a capacitor bank and inparallel coupling to said capacitor bank a controllable inductor with amagnetic core, a main winding for alternating current, and a DC-controlwinding for direct current, said DC-control winding for control of themagnetic flux set up by the main winding via orthogonal magnetization ofthe core.
 8. The use of a reactive power compensating means forcompensation of reactive power in an electric network according to claim7, wherein the reactive power compensating means is coupled to the pointof common connection.
 9. The use of a reactive power compensating meansfor compensation of reactive power in an electric network according toclaim 7, wherein the transmission link comprises a high voltage step-uptransformer with its low voltage side coupled to the point of commonconnection and wherein the reactive power compensating means is coupledto said grid connection point at the load network.
 10. The use of areactive power compensating means for compensation of reactive power inan electric network according to claim 9, wherein the power generatingpart is located off-shore and wherein the transmission link comprises asub-marine cable.
 11. The use of a reactive power compensating means forcompensation of reactive power in an electric network according to claim7, wherein the load network has a short circuit capacity that is lowerthan 10 times the rated power of the power generating part.